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Spatially-Controllable Hot Spots for Plasmon-Enhanced Second-Harmonic Generation in AgNP-ZnO Nanocavity Arrays

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Spatially-Controllable Hot Spots for Plasmon-Enhanced Second-Harmonic Generation in AgNP-ZnO Nanocavity Arrays

Shaoxin Shen et al. Nanomaterials (Basel).

Abstract

Plasmon-enhanced second-harmonic generation (PESHG) based on hybrid metal-dielectric nanostructures have extraordinary importance for developing efficient nanoscale nonlinear sources, which pave the way for new applications in photonic circuitry, quantum optics, and biosensors. However, the relatively high loss of excitation energies and the low spatial overlapping between the locally enhanced electromagnetic field and nonlinear materials still limit the promotion of nonlinear conversion performances in such hybrid systems. Here, we design and fabricate an array of silver nanoparticle-ZnO (AgNP-ZnO) nanocavities to serve as an efficient PESHG platform. The geometry of AgNP-ZnO nanocavity arrays provides a way to flexibly modulate hot spots in three-dimensional space, and to achieve a good mutual overlap of hot spots and ZnO material layers for realizing efficient SH photon generation originating from ZnO nanocavities. Compared to bare ZnO nanocavity arrays, the resulting hybrid AgNP-ZnO design of nanocavities reaches the maximum PESHG enhancement by a factor of approximately 31. Validated by simulations, we can further interpret the relative contribution of fundamental and harmonic modes to Ag-NP dependent PESHG performances, and reveal that the enhancement stems from the co-cooperation effect of plasmon-resonant enhancements both for fundamental and harmonic frequencies. Our findings offer a previously unreported method for designing efficient PESHG systems and pave a way for further understanding of a surface plasmon-coupled second-order emission mechanism for the enhancement of hybrid systems.

Keywords: finite-difference time-domain; hybrid nanostructure; plasmonics; second-harmonic generation.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Fabrication procedures of arrays of ZnO nanocavities (ZCA), AgNP (on)-ZnO nanocavities (SZCA), and AgNP (in)-ZnO nanocavities (ZSCA).
Figure 2
Figure 2
Experimental setups for the second-harmonic generation (SHG) measurement.
Figure 3
Figure 3
Top-view SEM images of (a) ZCA, (b) SZCA, and (c) ZSCA. The corresponding EDS spectra of (d) ZCA, (e) SZCA, and (f) ZSCA are also presented. The inset in (b) shows the side-view morphology of SZCA and the Cartesian coordinate system mentioned in the computational method. The inset in (c) refers to the inverted top-view morphology of ZSCA, indicating the trapping of AgNPs into ZnO nanocavities. The EDS spectra are collected from the regions marked with red boxes in (ac), and the insets in (df) show the element weights determined from the corresponding EDS spectra. Note that in order to observe the interior structures of ZSCA, arrays on the Si substrate were lifted off by conductive tape and then inverted ZSCA were characterized.
Figure 4
Figure 4
(a) Power-dependent plasmon-enhanced second-harmonic generation (PESHG): the emission intensity at second-harmonic (SH) wavelength increases quadratically with increased pumping power ranging from 0.8 mW to 2.0 mW. Inset: the measured PESHG intensity increases linearly with the square of pumping power (P2). (b) The comparison of power-dependent PESHG performances for ZSCA (black square dots), SZCA (red circle dots), and ZCA (blue triangle dots), respectively. Note that: the measured SHG intensity is a function of the excitation (average) power, which fits to a square dependence for all three samples. Error bars denote the deviation of the average SH signal intensity through multiple acquisitions from different spatial positions of the same sample. (c) Simulated near-field distributions of ZCA (up), SZCA (middle), and ZSCA (bottom) with a given excitation wavelength at 800 nm. All data are normalized.
Figure 5
Figure 5
(a) Ag-NP dependent PESHG performances for SZCA(10), SZCA(20), SZCA(30), and SZCA(40) at a given power value (P = 1.4 mW). Insets: Ag-NP dependent SEM images of SZCA(10), SZCA(30), and SZCA(40), respectively. The red arrow refers to the trend of the coarsening of the AgNP size both in SZCA and ZSCA. The data in (a) are normalized by the maximum value of SZCA(30). (b) Ag-NP dependent PESHG performances for ZSCA(10), ZSCA(20), ZSCA(30), and ZSCA(40) at a given power value (P = 1.4 mW). Insets: Ag-NP dependent SEM images of ZSCA(10), ZSCA(30), and ZSCA(40), respectively. The data in (b) are normalized by the maximum value of ZSCA(40). Error bars in (a,b) denote the deviation of the average signal intensity through multiple acquisitions from three different spatial positions of the same sample. The red dashed lines are to guide the eyes. Simulated near-field distributions of (c) ZSCA(10) and ZSCA(30); (d) SZCA(10) and SZCA(30) were excited by an 800-nm pumping wavelength in logarithmic scale. All data are normalized.
Figure 6
Figure 6
(a) Simulated reflectance spectra for ZSCA(10) and ZSCA(20). Simulated near-field distributions of Ag-NP dependent PESHG-EF: (b) harmonic mode (400 nm) for ZSCA(10) and ZSCA(20); (c) fundamental mode (800 nm) for ZSCA(10) and ZSCA(20) at the logarithmic scale.

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